Fuel cells combine hydrogen and oxygen to produce electricity, and are quieter and more efficient than standard diesel generators. Currently available fuel cells typically utilize fuels such as hydrogen, methanol, or reformed natural gas. JP-8 is a common military jet fuel containing significant amounts of sulfur, which can poison the catalysts used in the fuel reformer and fuel cell. A convenient, efficient method for removal of sulfur compounds from JP-8 and other jet fuels such as Jet A is desirable, for example, for portable/mobile use in fuel cells.
Sulfur in hydrocarbon fuels is mainly present as polynuclear heterocyclic compounds. In conventional hydrodesulfurization (HDS) reactions, the most common industrial sulfur removal process, the sulfur compound benzothiophene and its derivatives are hydrogenated to thiophane derivatives before removal of the sulfur atom. Conventional HDS is catalyzed by promoted molybdenum sulfide, MoS2. Thiols, sulfides, thiophenes and unsubstituted dibenzothiophenes (DBTs) are relatively rapidly converted by HDS. However, the substituted DBTs are less readily converted. C. H. Bartholomew and Robert J. Farrauto, “Fundamentals of Industrial Catalytic Processes,” John Wiley & Sons, Inc., 2005.
Conventional hydrodesulfurization (HDS) is also capital and energy intensive. A typical industrial process of fuel HDS includes steps of 1) fuel compression to ˜100 atmospheres and mixing with compressed hydrogen; 2) mixture preheating to ˜350° C.; 3) exothermic reaction in three reactors with increasingly higher surface area; 4) heat removal; 5) processing in a high pressure separator in which light gases, e.g., H2, H2S, and low-molecular-weight hydrocarbons are removed; 6) liquid scrubbing from H2S and low-molecular-weight hydrocarbons in a low pressure separator; and 7) hydrogen recovery from byproduct and recycling. Bartholomew et al., 2005. Nevertheless, the concentration of sulfur compounds in hydrocarbon fuels must be reduced by more than 95%, requiring “deep desulfurization,” to meet the present requirements for fuel sulfur content, and/or meet SO2 emissions standards.
Therefore, a convenient, efficient, alternative method for removal of sulfur compounds from hydrocarbon fuels is desirable for mobile and portable applications, as well as stationary applications such as at an oil refinery.
Various alternative methods to HDS for hydrocarbon fuel desulfurization have been disclosed.
Distillation is one conventional method for separating two or more liquid compounds on the basis of boiling-point differences. Distillation does not extract pure compound, especially if boiling points of the target compounds are close. Fractional distillation, which is also referred to as “rectification,” is a much more efficient separation process, which is the basis of many industrial processes including oil refinery and air separation. In addition, some closely boiling miscible fluid mixtures can form an azeotrope (constant boiling mixture), which requires addition of an entrainer for efficient separation by distillation processes.
Namazian et al., U.S. Pat. No. 7,303,598, Dec. 4, 2007, disclose a process for fractionating hydrocarbon fuel into light and heavy fractions in a fuel preprocessor (FPP). The light fraction is optionally further desulfurized by adsorption in an organic sulfur trap (OST), or by a hydrodesulfurizer step, and then reformulated in a steam reformer into a reformed fuel appropriate for use in fuel cells. Namazian Table 2 illustrates that by removing 30% heavy ends from JP-8 fuel by fractionation, the amount of sulfur is reduced by 50% to 371 ppm with 45% loss of polyaromatics. Disadvantages of fractionation by FPP include the need for fuel reformulation, loss of significant amount of fuel as heavy ends, and moderate ability to remove sulfur.
Ma et al. used adsorptive desulfurization of JP-8 jet fuel and its light fraction over nickel-based adsorbents for fuel-cell applications. However, this technique is limited by adsorbent capacity. See Ma et al., “Adsorptive desulfurization of JP-8 jet fuel and its light fraction over nickel-based adsorbants for fuel cell applications,” Prep. Pap, Am. Chem. Soc. Div. Fuel Chem. 2003, 48(2), 688.
Velu et al. used various zeolite-based adsorbants for removing sulfur from jet fuel, but this technique is also limited by finite sulfur adsorption capacities and selectivity for sulfur compounds compared to aromatics. Velu et al., Ind. Eng. Chem. Res. 2003, 42, 5293-5304.
Given the limitations of prior art methods, there is need for an efficient fuel desulfurization method that allows sulfur removal without significant fuel reformulation, substantially reduces capital and operational expenses associated with stationary fuel desulfurization, and permits portable and mobile fuel desulfurization applications.
An alternative technical approach utilizing vortex tube separation of mixtures of miscible liquids is provided herein. The vortex tube approach is applicable to removal of sulfur compounds from hydrocarbon fuels, and more broadly applicable to any process that requires separation of fluids with close boiling temperature.
Use of vortex tubes is proven to support rectification processes, particularly, air separation on nitrogen-rich and oxygen-rich streams. Bennett et al., U.S. Pat. No. 5,305,610, Apr. 26, 1994, provides a vortex tube process for producing nitrogen and oxygen. G. I. Voronin et al., “Process and Apparatus for Producing Nitrogen and Oxygen,” U.S. Pat. No. 4,531,371, Jul. 30, 1985; D. L. Bennett et al., “Process and Apparatus for Producing Nitrogen and Oxygen”; and V. Balepin, Ph. Ngendakumana, and S. Gauthy, “Air Separation with the Vortex Tube: New Experimental Results,” AIAA-98-1627, 1998. Representative additional patents include U.S. Pat. Nos. 1,952,281; 3,546,891; and 6,936,230.